In recent years, techniques relating to apparatuses built into computers and peripheral apparatuses connected to computers from the outside have been developing rapidly, in accordance with the development of computer technique. One such technique relates to information storage devices having a flat information storage medium, such as a magnetic disc, which store information by writing in information in the information storage medium.
Some information storage devices record information in an information storage medium and reproduce (access) information from the storage medium by moving a head which functions to record and reproduce information over the information storage medium while the latter is rotated. A hard disc device (HDD) is a typical example of such an information storage device. In most information storage devices in which a head accesses an information storage medium, a number of data regions for writing in and reading out user information (hereinafter simply referred to as “data”) handled by the user and a number of servo regions for storing information for positioning the head (hereinafter simply referred to as “position information”) are formed as different regions on the information storage medium in the device, so that the respective data regions can be identified by means of the position information stored in the servo regions. When the head reads out position information from the servo regions, the control section, which controls the head, identifies the position of the head over the storing medium, so that the head can be positioned above a desired data region. Thus, information storing media where data regions and servo regions are regularly aligned according to a predetermined positional relationship are ideal information storing media for positioning the head.
Here, servo regions and data regions are described more specifically citing a magnetic disc built into a hard disc device (HDD).
FIG. 1 is a diagram illustrating an ideal magnetic disc 1000, where data regions and servo regions are regularly aligned according to a predetermined positional relationship.
As illustrated in this figure, data regions 1002 in stripe-shaped, each of which is provided between two servo regions 1001, extend straight in the direction of the circumference of the magnetic disc 1000 (horizontal direction in figure). The head moves the right in the figure relative to the magnetic disc 1000 when the magnetic disc 1000 rotates (that is, the magnetic disc 1000 moves to the left when rotating), and records/reproduces (accesses) information in the magnetic disc 1000 while moving. Here, pairs of servo regions 1001 and data regions 1002 to the right are referred to as sectors, and sectors aligned in the direction of the circumference of the magnetic disc 1000 and running around the center of the disc form tracks 1005. The respective servo regions 1001 store information on the position on the magnetic disc 1000 for each servo region 1001, and this information identifies the position of the head (specifically, the position of the reproducing element or recording element in the head for accessing information), namely, at which track the head is, at which sector, or how far from the center position of the servo regions 1001 in the direction of the radius of the magnetic disc 1000. Data regions 1002 and servo regions 1001 are provided with magnetic regions formed of a magnetic material, and data information and position information are recorded in a form of magnetization direction of magnetization formed in the magnetic regions of the data regions 1002 and the servo regions 1001. Here, nonmagnetic regions 1003 formed of a nonmagnetic material (material having extremely low susceptibility) are provided in the top and bottom side of the data regions 1002 in the figure, and thus, the data regions 1002 are discretely aligned in the direction of the radius of the magnetic disc 1000 (vertical direction in figure) with servo regions 1001 in between, so that the data regions can be clearly distinguished in the direction of the radius of the magnetic disc 1000 when the head stores/reproduces (accesses) information. Here, the servo regions 1001 are illustrated as being aligned in the vertical direction, and perpendicular to the direction in which the tracks extends (horizontal direction in figure) in the figure. This is because only a part of the configuration of the magnetic disc 1000 is illustrated, and the servo regions 1001 are actually aligned slightly diagonally relative to the direction in which the tracks extend (horizontal direction in figure) when the magnetic disc is viewed in its entirety (see FIG. 7 below) This is the same in the following FIGS. 2 and 3.
In the work of forming the regions in the magnetic disc 1000 as illustrated in FIG. 1, first a number of annular regions to be the basic form of tracks and run around the center of the disc are formed in the direction of the radius of the magnetic disc with nonmagnetic regions in stripe-shaped which run around the center of the disc in between. Next, a number of line regions are formed so as to extend in lines from the center of the magnetic disc toward the edge of the magnetic disc and divide the annular regions. Here, annular regions sandwiched between two line regions become the data regions 1002 in FIG. 1. Each line region is divided into unit storage regions for storing position information indicating the position on the magnetic disc, and position information is recorded in each unit storage region. The unit storage regions, where position information is recorded, are the servo regions 1001 illustrated in FIG. 1.
As described above, in ideal magnetic discs 1000, data regions 1002 in stripe-shaped extend straight in the direction of the circumference of the magnetic disc 1000 (horizontal direction in figure). In addition, in ideal magnetic discs 1000, the centers of the servo regions 1001 adjacent to respective data regions 1002 aligned in the horizontal direction in the figure are in the same position relative to the direction of the radius of the magnetic disc 1000 (vertical direction in figure), and furthermore, are in the same position as the center of respective adjacent data regions 1002 relative to vertical direction in the figure. The center positions S1, S2, S3, S4, S5, S6 and S7 of the seven servo regions 1001 second from the top are in the same position in the vertical direction in the figure, and in the same position as the center positions D1, D2, D3, D4, D5, D6 and D7 of the data regions 1002 adjacent to respective servo regions in the vertical direction in the figure.
When the head accesses information, the position of the head is controlled on the basis of the information on the position of the head at that time as read out by the head from the servo regions 1001, so that the position of the head (specifically, the position of the reproducing element or the recording element in the head) becomes the center position of respective servo regions (for example the center positions S1, S2, S3, S4, S5, S6 and S7 in FIG. 1). Specifically, the feedback control is executed so that the difference between the head position and the center position of the servo region 1001, which is identified by readout of the servo region 1001 at the time when the head region passes through the servo region 1001, becomes zero. And thereby, the head position coincides with the center position of each servo region 1001. As a result of this control, the head accesses information along the center line, which passes through the center of each data region 1002 adjacent to each servo region 1001 (for example the dotted line in FIG. 1) in ideal magnetic discs 1000, and thus, precise recording/reproducing of information is realized.
In reality, however, it is difficult to provide a magnetic disc where the data regions 1002 extend straight in the direction of the circumference of the magnetic disc 1000 and the center position of respective servo regions coincides each other in the direction of the radius of the magnetic disc 1000, as in ideal magnetic discs 1000, as illustrated in FIG. 1. This is because errors easily occur in the manufacture of magnetic discs. For example, the annular regions sometimes become undulated in the process of forming the annular regions, and as a result, data regions undulate. In addition, in the step of dividing the line regions into the unit storage regions (that is, the servo regions) position information in the respective unit storage regions into which the line regions are divided, the position where position information is recorded may shift, and as a result, the center positions of two servo regions adjacent to the same data region may shift to each other in the direction of the radius of the magnetic disc.
FIG. 2 is a diagram illustrating a magnetic disc 2000 where the data regions undulate and servo regions shift in position.
In the magnetic disc 2000 illustrated in FIG. 2, the respective data regions 2002 do not extend straight in the direction of the circumference of the magnetic disc 2000 in FIG. 2 (horizontal direction in figure), and thus undulate. In addition, the center positions are different from each other for the respective servo regions 1001 aligned in the horizontal direction in the figure.
When the position of the head is controlled so that it comes to the center positions of the respective servo regions 1001 with respect to the magnetic disc 2000, where the data regions undulate and the center position of the servo regions shifts in position as described above, the track of the head deviates from the data regions and enters into the nonmagnetic region 2003. When the head moves to the right in FIG. 2 relative to the data region 2002 second from the top in the magnetic disc 2000 in FIG. 2 in order to access information, for example, the head moves in such a manner that the center positions P1, P2, P3, P4, P5, P6 and P7 of the seven servo regions 1001 becomes the position of the head, and the track 5101 largely undulates in the vertical direction, as illustrated in FIG. 2. At this time, the data regions 2002 undulate, and thus, as illustrated in FIG. 2, the track 5101 of the head largely deviates from the data regions 2002 and enters deep into the nonmagnetic region 2003 in some portions. In this state, it is difficult to record/reproduce information with precision.
In order to prevent this, a new region where position correcting information is recorded to indicate the amount of position shift in the direction of the radius of the magnetic disc between the center position of the data region and the center position of the servo region is provided between servo regions and data regions, and an HDD for correcting the position of the head on the basis of the amount of position shift as read out from this region has been proposed (see, for example, Japanese Laid-open Patent Publication No. 2006-031846).
FIG. 3 is a diagram illustrating a magnetic disc 3000 having regions where position correcting information is stored, and FIG. 4 is a block diagram illustrating the control of positioning of the head in an HDD which adopts this magnetic disc 3000.
In the magnetic disc 3000 adopted in the HDD in Japanese Laid-open Patent Publication No. 2006-031846 illustrated in FIG. 3, correction information regions 3004 are provided between the date regions 3002 and the servo regions 3001 in FIG. 3. In this magnetic disc 3000, sets of servo regions 3001, correction information regions 3004 and data regions 3002 form individual sectors, and these sectors are aligned in the direction of the circumference of the magnetic disc 3000 and run around the center of the disc, and thus, tracks are formed. Position correcting information indicating the amount of position shift in the direction of the radius of the magnetic disc 3000 between the center position of the data region 3002 adjacent to a correction information region 3004 and the center position of the servo region 3001 adjacent to this correction information region 3004 is recorded in the same correction information region 3004. In the correction information region 3004a in the center in FIG. 3, for example, position correcting information indicating the amount of position shift ho between the center position Do of the data region 3002 to the right of this correction information region 3004 and the center position So of the servo region 3001 to the left of the same correction information region 3004a is recorded in the correction information region 3004a in the center in FIG. 3. In the HDD in Japanese Laid-open Patent Publication No. 2006-031846, a table for correcting the position (position correcting table) which illustrates the amount of position shift as read out from the respective correction information regions 3004 corresponding to the data regions 3002 adjacent to the respective correction information regions 3004 is prepared when the magnetic disc 3000 is incorporated in the HDD and stored in the memory 4000d in FIG. 4.
In the HDD in Japanese Laid-open Patent Publication No. 2006-031846, a system in which feedback control is carried out so that the difference between the position of the head and the center position of the data region 3002 in the direction of the radius of the magnetic disc 3000 becomes zero is adopted instead of the system where feedback control is carried out so that the difference between the position of the head and the center position of the servo region 3001 in the direction of the radius of the magnetic disc 3000 becomes zero. Specifically, as illustrated in FIG. 4, first, the difference s (s=rs−y) between the distance y between the center of the magnetic disc and the position of the head at that time and the distance rs between the center of the magnetic disc and the center position of the servo region 3001 is read out from this servo region 3001. Next, a feed forward (FW) control section 4000c finds the difference h (h=rd−rs) between the distance rs between the center of the magnetic disc and the center position of the servo region 3001 and the distance rd between the center position of the data region 3002 facing this servo region 3001 and the center of the magnetic disc with the correction information region 3004 intervening in between using the position correcting table stored in the memory 4000d in the feed forward control section 4000c and transfers the difference h to a correction adder 4000a. The correction adder 4000a adds the transferred difference h to the difference s between the distance y to the position of the head and the distance rs to the center position of the servo region 3001, and thus, the difference t (t=s+h=rd−y) between the distance y to the position of the head and the distance rd to the center position of the data region 3002 is found. Next, the feedback (FB) control section 4000b finds the feedback control value u for feedback control which makes the difference t zero, and a voice coil motor which moves the head is controlled using this feedback control value u. The feedback control value u is, specifically, a value of a current supplied to the voice coil motor. When the voice coil motor is controlled in this manner the head moves to a position at a distance y from the center of the disc. In this figure, the voice coil motor and the head are collectively represented as a plant 5000, and the plant 5000 outputs the distance y when the feedback control value u is inputted in the flow of control. After the head has moved, it reads out the servo region 3001 for destination so as to find a new difference s between a new distance y to the position of the head and a new distance rs to the center position of the servo region 3001, and the same control as described above is carried out on the basis of this difference. When this control is repeated, the position of the head y approaches the center position rd of the data region 3002 as time elapses.
As a result of this control, the head moves, drawing a track following the data regions 3002 aligned in the horizontal direction in FIG. 3 without entering deep into the nonmagnetic region 1003, as the track 5102 in FIG. 3, even when the data regions 3002 undulate.
In the system described in Japanese Laid-open Patent Publication No. 2006-031846, however, it takes time to make the difference t between the distance y to the position of the head and the distance rd to the center position of the data region 3002 zero through feedback control in magnetic discs 3000 where the difference h between the distance rs to the center position of the servo region 3001 and the distance rd to the center position of the data region 3002 is largely different in each data region 3002, that is, in magnetic discs 3000 where the distance between the position of the servo region 3001 and the position of the data region 3002 fluctuate in a high frequency in the direction of the radius of the magnetic disc, and as a result, the position of the head cannot sufficiently be prevented from fluctuating at the time of access.
FIG. 5 is a diagram illustrating the effects of correcting the position using a position correcting table in the case where the difference h between the distance rs to the center position of the servo region 3001 and the distance rd to the center position of the data region 3002 fluctuates with a period of one fifth of the length of the track in this position, and FIG. 6 is a diagram illustrating the effects of correcting the position using a position correcting table in the case where the difference h between the distance rs to the center position of the servo region 3001 and the distance rd to the center position of the data region 3002 fluctuates with a period of one tenth of the length of the circumferential length of the track in this position.
For the sake of simplicity, FIGS. 5 and 6 illustrate the results of simulation in the case where only the data regions undulate and there is no shift of position in the center position of the servo regions, as illustrated in FIGS. 2 and 3, in the magnetic disc.
Part (a) of FIG. 5 and part (a) of FIG. 6 are graphs illustrating how the difference t (hereinafter referred to as position error t) between the distance y to the position of the head and the distance rd to the center position of the data region 3002 changes in magnetic discs 3000 where 220 data regions 3002 are aligned per track in the direction of the circumference of the disc (that is, in magnetic discs 3000 where the number of sectors per track is 220) with the number of sectors defined in the lateral axis. Here, the position error t is the ratio (%) relative the width of the track.
In the graphs of part (a) of FIG. 5 and part (a) of FIG. 6, solid lines represents the position error t in the case where no position correcting table is used, and dotted lines represents the position error t in the case where the position is corrected using the respective position correcting tables illustrated in part (b) of FIG. 5 and part (b) of FIG. 6.
The position correcting tables in part (b) of FIG. 5 and part (b) of FIG. 6 respectively correspond to the graphs for the change in the difference h between the distance rs to the center position of the servo region 3001 (the distance rs has a constant value in this simulation) and the distance rd to the center position of the data region 3002, and illustrate the position correcting values for the respective sectors that are included in the position correcting table with the ratio (%) relative to the track width as the unit.
As illustrated in part (a) of FIG. 5, in the case where the difference h between the distance rs to the center position of the servo region 3001 and the distance rd to the center position of the data region 3002 fluctuates in a high frequency with a period of one fifth of the circumferential length of the track in that position, the graph having dotted lines, where the position is corrected using the position correcting table, fluctuates with a slightly smaller amplitude, than in the graph having solid lines, where the position is not corrected, and thus, it can be seen that the device can be improved, though only slightly, by correcting the position using the position correcting table.
In addition, as illustrated in part (a) of FIG. 6, in the case where the difference h between the distance rs to the center position of the servo region 3001 and the distance rd to the center position of the data region 3002 fluctuates in a high frequency with a period of one tenth of the circumferential length of the track in that position, the graph having dotted lines, where the position is corrected using the position correcting table fluctuate with a larger amplitude, than in the graph having solid lines, where the position is not corrected, and thus, it can be seen that the head actually ends up fluctuating more when the position is corrected using the position correcting table. This is because the feedback control cannot follow promptly enough in when vibrating with a high frequency, and therefore, the feedback control to amplify the oscillation instead of suppressing it.
As described above, the position of the head cannot sufficiently be prevented from oscillating when the head accesses information in the magnetic disc 3000, where the distance between the position of the servo region 3001 and the position of the data region 3002 fluctuates in a high frequency in the direction of the radius of the magnetic disc in the system described in Japanese Laid-open Patent Publication No. 2006-031846, and thus, a problem arises, such that it is difficult to record/reproduce information with precision.
Though an HDD is exemplified for the description above, the problem described above is not limited to HDD, and could arise with other general information storage devices in which the position of the head is determined by reading out information from servo regions.